The present invention pertains to the field of polymer chemistry and specialty chemicals. More specifically, the present invention pertains to the field of polymer fibers and the specialized chemicals associated with developing dyed polymer fibers. In particular, this invention relates to the production of salt derivatives of isophthalic acid, specifically the production of an alkali metal salt derivative, such as a mono-lithium salt of 5-sulfoisophthalic acid, for use in the production of dyed nylon fibers among other polymer fibers.
Although derivatives of 5-sulfoisophthalic acid are used in several polymer processes this discussion will focus on nylon to aid in the understanding of the invention. The narrative and examples presented herein are for purposes of explanation and not of limitation.
Many types of nylons exist and are usually differentiated based on the components used to make them. Generally speaking, nylons are made by reacting equal parts of a diamine with a dicarboxylic acid. The particular diamine and acid used in the reaction gives the nylon its name. For example, “nylon 6-6” is a term used to identify nylon made by reacting hexamethylene diamine and adipic acid. Both components donate 6 carbons to the polymer chain thus the nylon is designated “6-6”.
Nylon fibers, especially those used for carpet fiber, are also classified as to type, depending on the fiber's receptivity to acid dyes and basic or cationic dyes. Cationic dyeable nylon fiber generally exhibit inherent stain resistant properties as compared to other nylon types but traditionally suffered from poorer lightfastness, especially in light shades. This resulted in the under-utilization of cationic dyeable nylon as a carpet fiber.
As expected, considerable time, energy, and resources were devoted to finding new and improved methods to enhance the dye absorbing characteristics of cationic dyeable nylon. Over the years, several methods were developed in which very specialized chemicals were added to the fiber production process to impart improved cationic dye-ability to the polymer. One such specialized chemical is the lithium salt of 5-sulfoisophthalic acid, commonly known as LiSIPA.
The currently utilized processes for the production and purification of LiSIPA have numerous disadvantages, including a low product yield, colored product, and high manufacturing costs. Further, the resultant LiSIPA from known processes typically has a high sulfate level (i.e., above 500 ppm). More typically the sulfate levels in LiSIPA from known processes ranges from 1000 to 3000 ppm.
A problem inherent to the production of high sulfate LiSIPA is that the sulfate can precipitate in the fiber production process. Sulfate precipitation can lead to high levels of nylon filament breakage and lost production. It is believed that known LiSIPA products undergo additional treatment to reduce sulfate levels. However, such treatment increases production costs.
Another problem inherent to the high sulfate LiSIPA's produced by current methods is that there are limited means of removing the sulfate. For example, some of the sulfate can be removed by washing the LiSIPA with water or re-crystallizing the LiSIPA in water. Unfortunately, LiSIPA is soluble in water. Thus, using water for sulfate removal results in lost product.
Due to these and other problems in the prior art, some of which are disclosed herein, there is a need for a method of producing a LiSIPA product having inherently low sulfate content. In other words, there is a need for a method of producing LiSIPA that results in a LiSIPA product that has low levels of sulfate without any further treatment beyond simple collection and washing. The method should be suitable for commercialization using equipment currently employed in most LiSIPA manufacturing processes.